There have been conventional liquid crystal display devices in which a thin-film transistor (hereinafter, TFT) made of amorphous silicon (hereinafter, a-Si) or polycrystalline silicon (hereinafter, P-Si) is formed on a glass substrate, which are used for driving liquid crystal display panels and organic EL panels, i.e. for performing active matrix drive.
In particular, liquid crystal display devices in which peripheral drives are integrated using p-Si that has high mobility and operates at a high speed have recently been used. Even so, a silicon device with improved performance is required for realizing system integration of devices such as an image processor and a timing controller which are required to have performance better than the above-mentioned peripheral devices.
This is because, when adopting P-Si, the mobility is decreased and an S (subthreshold) factor is increased due to localized states in a gap caused by the incompleteness of crystallinity and the deficiency around a grain boundary, so that the transistor does not have sufficient performance to form a high-performance silicon device.
Thus, to form a silicon device having higher performance, research has been done in which a device such as a thin-film transistor made of single-crystal silicon thin film is formed in advance, and then a semiconductor device is manufactured by bonding this thin-film transistor on an insulating substrate (e.g. WO93/15589 (published on Aug. 5, 1993), J. P. Salerno, “Single Crystal Silicon AMLCDs”, Conference Record of the 1994 International Display Research Conference(IDRC) P. 39-44(1994), and Q.-Y. Tong & U. Gesele, “SEMICONDUCTOR WAFER BONDING: SCIENCE AND TECHNOLOGY”, John Wiley & Sons, New York (1999)).
WO93/15589 teaches that, using a semiconductor device in which a single-crystal silicon thin-film transistor formed in advance using an adhesive is printed on a glass substrate, a display of a display panel of an active matrix liquid crystal display device is manufactured.
However, according to this conventional semiconductor device and the manufacturing method thereof, since the adhesive is used for bonding the single-crystal silicon thin-film transistor, which has high performance, with the glass substrate, the bonding operation is cumbersome and hinders the improvement of productivity. Further, being bonded using the adhesive, the semiconductor device has low heat resistance, and it is not possible to form high-quality members such as an inorganic insulating film and a TFT in the subsequent process. For this reason, on the occasion of manufacturing an active matrix substrate, it is necessary to form a device including a TFT array before bonding the device to a substrate, and this has been a great disadvantage in terms of a cost/size ratio and formation of a wiring.
Moreover, WO93/15589 only teaches that a single-crystal silicon thin film device is formed on a glass substrate, and according to this arrangement, it is not possible to manufacture a high-performance/high-quality semiconductor device which has been sought after.
K. Warner, et al., 2002 IEEE International SOI Conference: October, pp. 123-125 (2002) teaches that an aligning mark is detected over a silicon substrate by means of infrared light so that aligning is performed. However, with this arrangement, it is not possible to increase resolution due to long wavelength of the light, and hence high-precision aligning cannot be performed.
Further, L. P. Allen et al., 2002 IEEE International SOI Conference: October, pp. 192-193 (2002) teaches that a silicon on a BOX (Buried Oxide) is evenly etched by a halogen gas cluster ion beam (GCIB) made of about 1500 atoms, and a high-frequency component of surface roughness on the surface of the silicon is removed by GCIB using oxygen.
Now, another problem which has conventionally been known will be discussed. A thin-film transistor (TFT) technology relates to manufacture of a transistor by forming a semiconductor film such as a silicon film on, for instance, a light-transmitting amorphous material such as a glass substrate. This TFT technology has been developed in line with the diffusion of personal intelligent communicators using liquid crystal display panels.
In the TFT technology, a polysilicon (polycrystalline) film is formed by, for instance, melting an amorphous silicon film on a substrate by applying laser thereto. Then from this polysilicon film or amorphous silicon film, a MOS TFT as a switching element is manufactured.
In this manner, using a device (MOS TFT) made from a silicon film, display panels such as a liquid crystal display panel and organic EL panel are manufactured.
Then pixels of the display panel are driven in an active matrix manner by the MOS TFT.
This arrangement has been used for devices such as a TFT liquid crystal display (LCD) device, a TFT organic electro-luminescence (OLED: Organic Light Emitting Diode) display device.
To drove the switching elements in an active matrix manner, a silicon device with higher performance is required and system integration of devices such as a peripheral driver and a timing controller is necessary.
However, it is not possible to obtain the required high performance when a conventionally-used amorphous silicon film or a polycrystal film is adopted.
This is because, in the polycrystal silicon film and the like, there are localized states in a gap caused by the incompleteness of crystallinity and the deficiency around a grain boundary. The existence of the localized states decreases the mobility. Further, due to the increase of a subthreshold (S) factor, the performance of the transistor is caused to be insufficient so that a high-performance silicon device cannot be formed.
Moreover, when the crystallinity of the silicon film is insufficient, a fixed charge tends to be formed at the interface between silicon and a gate insulating film. On this account, it is difficult to control a threshold voltage of the thin-film transistor, thereby being impossible to obtain a required threshold voltage.
In the case of the TFT liquid crystal display, a polycrystalline silicon film is formed from an amorphous silicon film by means of methods such as heating using laser light. In this process, since the energy of the laser causes a certain degree of fluctuation, the particle size of the obtained polycrystalline silicon film is not consistent. On this account, the mobility and threshold voltage greatly vary.
When an amorphous silicon film formed by methods such as a plasma CVD (Chemical Vapor Deposition) is heated by laser light and then crystallized, a temperature of a surrounding area of the film promptly rises to be nearly a melting point of silicon. Thus, when a non-alkali high strain point glass is adopted as a substrate, substances such as alkaline metal are diffused into the silicon through the glass. For this reason, the characteristics of the transistor to be obtained deteriorate.
To solve this problem, a device using single-crystal silicon has been developed, in parallel with research for further homogenization and improvement of crystallinity of polycrystalline silicon.
An SOI substrate is an example of devices using such single-crystal silicon (SOI is an abbreviation of Silicon on Insulator). SOI technology for the SOI substrate mainly relates to formation of a single-crystal semiconductor thin film on an amorphous substrate. This term, SOI technology, is not frequently used in respect of formation of a polycrystalline silicon film. SOI technology has been actively developed since 1980s.
As an example of the SOI substrate, there is a SIMOX (Separation by Implanted Oxygen) substrate which has been commercially available. This SIMOX substrate is formed by implanting oxygen into a silicon wafer. In this process, since oxygen, which is a relatively heavy element, is implanted to a predetermined depth, a crystalline structure of the silicon wafer is seriously damaged due to an accelerating voltage involved in the implantation. Thus, the SIMOX substrate has such a problem that the characteristics of single-crystal on the substrate are not sufficient. Further, the insulation performance is inadequate due to non-stoichiometry of a silicon dioxide film layer, and since a large amount of oxygen is required for the implantation, the costs for the ion implantation is high.
In response to this, Japanese Laid-Open Patent Application No. 5-211128/1993 (Tokukaihei 5-211128; published on Aug. 20, 1993) discloses a method of manufacturing a thin semiconductor film, which is arranged such that a single-crystal silicon piece is bonded on a silicon base substrate covered with an oxidized silicon film, and the resultant substrate with the silicon piece is manufactured to be a thin film.
According to this technology, a single-crystal silicon thin film can be formed on a single-crystal silicon base substrate on which an oxidized film has been formed in advance.
Japanese Laid-Open Patent Application No. 2000-30996 (Tokukai 2000-30996; published on Jan. 28, 2000) discloses a standard deviation of thickness of an oxidized film on a silicon wafer, regarding an SOI wafer and a manufacturing method thereof.
Also, Japanese Laid-Open Patent Application No. 6-268183/1994 (Tokukaihei 6-268183; published on Sep. 22, 1994) discloses a method of manufacturing a semiconductor device, in which method a thinly-manufactured substrate on which a semiconductor device has been formed is transferred to another supporting substrate.
In this method, after a semiconductor element is formed on one side of a semiconductor layer, the semiconductor layer manufactured as a thin layer is bonded with a supporting substrate, by means of cold anode connection.
However, in this arrangement, micro-roughness of the oxidized silicon film on the substrate weakens the adhesive power so that film stripping occurs.
That is to say, according to Japanese Laid-Open Patent Application No. 5-211128/1993, the thickness of the oxidized film is significantly irregular when the film on the silicon base substrate is thickly formed. Thus, the irregularity of the surface becomes noticeable and the adhesiveness of the bonding and the characteristics of the SOI substrate deteriorate.
Note that, although Japanese Laid-Open Patent Application No. 2000-30996 includes the description regarding uniformity of the thickness of the single-crystal silicon thin film on occasions when the standard deviation of the thickness is large. However, the document does not mention the problems such as the formation of voids on the occasion of bonding and the stripping of the silicon film on the occasion of separation and stripping.
Further, Japanese Laid-Open Patent Application No. 6-268183/1994 does not describe the micro-roughness and flatness of the thinned semiconductor layer and the supporting substrate.
In this manner, the micro-roughness of the oxidized silicon film by which a light-transmitting substrate is covered weakens the adhesive power. On this account, separation and stripping occur so that the yield decreases due to reasons such as the film stripping after forming a silicon film on a substrate.